1. Field of the Invention
The present invention relates to a method of forming a polymer insulator, particularly relates to a forming method in which polymer insulators having various shed numbers can be formed without reconstructing a die used for forming.
2. Description of Related Art
A compression forming technique for forming a polymer insulator known. FIGS. 5a to 5d are schematic views showing successive steps of forming the polymer insulator by means of the known compression forming. At first, as shown in FIG. 5a, a formed body 34 is prepared. The formed body 34 has a construction such that a pre-form material 32 made of silicone rubber is arranged around an FRP rod 31 and end caps 33 are arranged at both end portions of the FRP rod 31. It should be noted that a winding amount of the pre-form material 32 is determined corresponding to the number of sheds. Moreover, a cavity having a predetermined shape is arranged at a side surface of the end cap 33 to which the pre-form material 32 is contacted. Then, the pre-form material 32 is filled in the cavity when performing the compression forming.
Then, as shown in FIG. 5b, the thus prepared formed body 34 is set between an upper die 41 and a lower die 42. Each of the upper die 41 and the lower die 42 has cavities 35 used for forming sheds and sheath portions of the polymer insulator and also cavities 36 used for accommodating the end cap 33. Generally, the upper die 41 and the lower die 42 are respectively constructed by integrating a plurality of segments 43, and, prior to the compression forming, they are constructed corresponding to the number of sheds of the polymer insulator to be formed. After that, as shown in FIG. 5c, the upper die 41 is pressed to the lower die 42 to perform the compression forming. After the compression forming, the upper die 41 is detached from the lower die 42, and further the end caps 33 are removed from the FRP rod 31 so as to obtain a polymer insulator formed body 51.
The thus obtained polymer insulator formed body 51 has an overcoat layer comprising sheds 52 and sheath portions 53, which is arranged around the FRP rod 31, and also has securing metal fitting connection portion 54 at its both end portions. Projection portions 55 are arranged on an entire circumferential surface of the securing metal fitting connection portion 54. These projection portions 55 are served as an 0 ring when a securing metal fitting not shown is connected to the securing metal fitting connection portion 54. Therefore, it is possible to improve seal performance of a connection boundary between the securing metal fitting and the overcoat layer.
As mentioned above, in the known method of forming the polymer insulator, in order to achieve a high seal reliability between the securing metal fitting and the securing metal fitting connection portion 54, it is necessary to work the securing metal fitting connection portion 54 precisely and to arrange the projection portions 55 on an outer surface of the securing metal fitting connection portion 54. Therefore, the compression forming must be performed after setting the end caps 33 at both ends of the FRP rod 31. In this case, it is possible to achieve working precision of the securing metal fitting connection portion 54. However, when the polymer insulator having a different shed number from that of the previous one is to be formed, the upper die 41 and the lower die 42 must be reconstructed corresponding to the shed number of the next polymer insulator.
Here, it is assumed that one polymer insulator is formed by using the upper die 41 (the lower die 42) shown in FIG. 6a and then the other polymer insulator having the shed number smaller than that of the previous polymer insulator is to be formed. In this case, in order to decrease the number of the cavities 35 and to change a position of the cavity 36 for the end cap 33, it is necessary to reconstruct the upper die 41 (the lower die 42) as shown in FIG. 6b by replacing the segments 43. Therefore, in the known method of forming the polymer insulator, there is a drawback such that a remarkable time is necessary for reconstructing the die.